This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. HIV-1 infections are acquired most often through sexual contact, with the majority of the sexual transmission of HIV-1 worldwide occurring as a result of heterosexual contact. Women of childbearing age are at the greatest risk for HIV-1 infection, resulting in a corresponding increase in HIV-1 infection in women, newborns, and infants worldwide. However, despite the predominance of sexual transmission in the continued spread of HIV-1, the mechanisms of sexual transmission of HIV-1 to women are still poorly understood. For example, it is not known whether cell-free virus, cell-associated virus or both are essential for HIV-1 transmission in humans. Potential mechanisms of HIV-1 transmission across mucosal epithelium include 1) direct infection of epithelial cells;2) transcytosis through epithelial cells and/or specialized microfold (M) cells;3) epithelial transmigration of infected donor cells;4) uptake of intraepithelial Langerhans cells and 5) circumvention of the epithelial barrier through physical breaches. Successful transfer of virus across epithelial barriers may result in HIV-1 uptake by migratory dendritic cells (by DC-SIGN or another mannose C-type lectin receptor) and subsequent dissemination to draining lymph nodes and/or localized mucosal HIV-1-infection, leading to recruitment of additional susceptible cells. Although many questions remain unanswered, these investigations have revealed potential targets for prevention of HIV transmission in women which are critical for limiting the global AIDS pandemic. We propose to evaluate therapeutic antibody gene transfer as a novel and durable anti-HIV microbicide using a bivalent human anti-hCXCR4 scFvFc "minibody" (human single-chain antibody linked in frame to the Fc (Hinge-CH2-CH3) domain of human IgG1) that potently inhibits X4-tropic HIV-1 infection and cross-reacts with macaque CXCR4. Published studies support the likelihood that high levels of therapeutic antibodies can be achieved for at least 4 months following a single gene transfer with these non-pathogenic viruses. AAV gene transfer vectors are rapidly moving into advanced stage human clinical trials for other indications. Importantly, similar studies will be performed with rhesus macaque cross-reactive anti-CCR5 scFvFcs as soon as these Abs are isolated and HIV/SIV-entry inhibition studies are completed.